Termly attendance module

Introduction

The configuration subsystem arranges each of the subsystem components on the spacecraft. A very useful tool for this arrangement is DrawCraft.8 The subsystem chairs publish the

dimensions, mass, and locations (if applicable) of each of the components using ICEMaker.

Next, these values are automatically updated to a SCMS (Shared Mechanical Control Sheet) text delimited file, which is then read by DrawCraft. DrawCraft then creates an assembly in

SolidWorks which provides information on the weight distribution and surface area over the entire spacecraft.

The components need to be placed in such a way that certain criteria are met. Since this spacecraft is traveling through a significantly dense part of the atmosphere, it needs to be aerodynamically stable. In this case, the center of gravity needs to be forward of the half-chord point. For our purposes, the length of the spacecraft is approximated as its chord. Also, since the scientific sensor suite was previously chosen, the requirements of the sensors need to be met.

The ADMS and CADS sensors are required to be ram-facing, and the SETA sensor is required to be within six inches of the center of gravity of the entire spacecraft. Another important

requirement is that the entire vehicle needs to be able to fit inside the payload fairing for the chosen launch vehicle.

This subsystem is built so that with a small amount of human involvement, the configuration of the satellite can be dynamically changed during the ICE sessions. Human involvement is required for several different reasons. First, the updated SCMS needs to be loaded into DrawCraft. Once DrawCraft creates the model in SolidWorks, one needs to open a special window within SolidWorks to produce the weight distribution and surface area outputs.

DrawCraft does provide some of these required outputs; however, SolidWorks provides all of the required outputs, and does so in a favorable manner. For example, the moments of inertia

calculated by DrawCraft are about a reference axis, and the moments created by SolidWorks are both around a reference axis and the center of gravity. Since the center of gravity changes with every design iteration, SolidWorks is a more useful tool. It is conceivable that this type of program interface could be automated so that the SCMS file is automatically updated, and the information is automatically published from SolidWorks. This would aid greatly in the speediness of the ICE sessions. It was not developed in this case because of time constraints.

Even if the SCMS file could be automatically updated, and the outputs automatically published, it would only take care of parametric variations on the design. The configuration subsystem is unique in that at each iteration step in the ICE session, the configuration needs to be visually evaluated and possibly changed by the configuration chair. A good example of this is that at the start of the ICE sessions, the original design for the fuel tank was a single sphere. As the fuel mass increased, the fuel tank impinged upon, then eventually exceeded, the wall of the main bus.

The result was that a non-parametric change to two cylindrical tanks needed to be made, as can

8 DrawCraft - Dr. Joel C. Sercel (Caltech, Pasadena, California, USA) in the Laboratory for Spacecraft and Mission Design for the use of the DrawCraft (a spacecraft configuration tool).

be seen below in figure 1. Another example that illustrates the necessity of configuration

evaluation at each step concerns the scientific sensors. Once the change to cylindrical fuel tanks was made, a trade was performed in which the satellite altitude was lowered. This required more fuel to be aboard, and the fuel tanks to lengthen. Eventually the tanks, though they fit inside the main bus, encroached upon the space needed for the scientific instruments. This can only be seen when the configuration chair takes the time to visually evaluate the design. In this trade, the constraining factor happened to be the space required for the fuel tanks. If the configuration were not evaluated visually, an impossible design could be chosen.

Figure 7–1: Final design. Note cylindrical fuel tanks (grey)

Inputs

Generally, the inputs to the configuration subsystem are the dimensions, mass, and location of each of the satellite components. The components that were modeled were:

• Main bus

• CADS, ADMS, and SETA sensors,

• Omni-directional antennas

• Primary and secondary batteries

• Fuel tanks

• ADACS thrusters

• Main Thruster

• Telecom boxes

• C&DH computers

Outputs

The parametric outputs of this subsystem described the weight, surface area, and volume of the total spacecraft. These are listed below:

• Basic cross-sectional shape of the main bus

• Basic shape of entire bus

• Cross sectional area

• Total surface area

• Coefficient of drag

• Distance from the c.g. to center of aerodynamic pressure

• Distance from the c.g. to center of solar radiation pressure

• Distance from the total internal torque to c.g.

• Moment of inertia, mass xx

• Moment of inertia, mass xy

• Moment of inertia, mass xz

• Moment of inertia, mass yy

• Moment of inertia, mass yz

• Panel area

• Total volume

Another important product of this subsystem is a CAD drawing that provides information on the placement of each component. SolidWorks drawings of the selected architecture can be found in Appendix A.

Assumptions

The limit of the coefficient of drag on a blunt body in the upper atmosphere, computed using free-molecule flow, is found to be 2.0.9 Since extensive modeling would be required in order to produce a more accurate number, this value is used as a constant throughout the design. A sensitivity analysis should have been performed on this value, but was not due to time considerations.

Another important assumption is that the antennae can be folded along the main bus in order for the spacecraft to fit inside the launch vehicle payload fairing.

In order to distribute the mass of the structures and mechanisms (cabling, small struts, etc), it is contained in the mass of the main bus, which is evenly distributed along the length of the bus.

It is also assumed that solar arrays would be able to be attached to the body of the main bus.

Fidelity Assessment and Verification

The fidelity of the parametric outputs is only as accurate as the inputs used to generate them.

Since all of the inputs are physical parameters of subsystem components, the fidelity of the model depends on the combined fidelity of all of the contributing subsystems. Some of the

9 Hoerner, Sighard, Fluid-Dynamic Drag, Sighard Hoerner, 1965

outputs, however, such as the distances, are approximate values, derived from the SolidWorks configuration. These are approximated since those values do not change appreciably during the ICE session iterations, and it is costly time-wise to input these values at each iteration. The SolidWorks model directly reflects the inputs from each of the subsystems.

An electronic copy of the configuration subsystem sheet can be found on the XTOS compact disc (Configuration.xls).